1. Field of the Invention
The present invention relates to liquid crystal active matrix displays, and more particularly concerns a liquid crystal display module drive voltage that is periodically reversed without the use of large numbers of switching amplifiers.
2. Description of Related Art
Liquid crystal display devices require repetitively reversed voltage to be applied across the counter electrodes that are provided on opposite sides of the multi-layer liquid crystal display (LCD) module. For the active matrix LCD, shades of grey are produced by applying different levels of drive voltage to achieve a desired intensity of the image. The different voltage levels are provided by digital to analog converters (DAC's). Because of the requirement for alternating polarity drive signals, which provide a zero average voltage across the liquid crystal material, the drive signal level, which includes the analog output of the DAC, is usually reversed in alternate lines and/or in alternate frames by use of an amplifier that is switched between inverting and noninverting modes. If a design requires 64 DAC's, for example, then 64 such switching amplifiers are needed to perform the inversion function. Such switching amplifiers are more complex, more expensive, subject to undesirable voltage drift and require closely matching tolerances. At least partly because the switching function adds capacitance to the amplifier circuit, the switching amplifier must be made to operate at higher speed to accommodate the slowing effect of the capacitance. Temperature compensation is highly desirable for LCD's because of the sensitivity of the liquid crystal material operation to temperature variation. With the use of an amplifier that switches between inverting and noninverting modes, temperature tracking and temperature compensation are more difficult.
Accordingly, it is an object of the present invention to provide a periodically reversing voltage drive for a liquid crystal active matrix LCD that avoids or minimizes above mentioned problems.